feat(notifications): implement /notifications page with filter pills and load-more

New route with server load function (reads URL params, derives unreadCount from
the page, single API call per Sara's architecture requirement), mark-all form
action, and the full page UI: filter pills with ARIA radiogroup, notification
rows with border+dot unread indicators (WCAG 1.4.1), "Ältere laden" client-side
append, and empty state. Includes all de/en/es translation keys.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/notifications/+page.server.ts

@@ -0,0 +1,35 @@
+import { error, redirect } from '@sveltejs/kit';
+import { createApiClient } from '$lib/api.server';
+import { getErrorMessage } from '$lib/errors';
+import type { PageServerLoad, Actions } from './$types';
+
+export const load: PageServerLoad = async ({ fetch, url }) => {
+	const api = createApiClient(fetch);
+
+	const type = url.searchParams.get('type') ?? undefined;
+	const readParam = url.searchParams.get('read');
+	const read = readParam !== null ? readParam === 'true' : undefined;
+
+	const result = await api.GET('/api/notifications', {
+		params: { query: { type: type as 'MENTION' | 'REPLY' | undefined, read, page: 0, size: 20 } }
+	});
+
+	if (!result.response.ok) {
+		const code = (result.error as unknown as { code?: string })?.code;
+		throw error(result.response.status, getErrorMessage(code));
+	}
+
+	const page = result.data!;
+	const notifications = page.content ?? [];
+	const unreadCount = notifications.filter((n) => !n.read).length;
+
+	return { notifications, unreadCount, totalPages: page.totalPages ?? 1 };
+};
+
+export const actions: Actions = {
+	'mark-all': async ({ fetch }) => {
+		const api = createApiClient(fetch);
+		await api.POST('/api/notifications/read-all');
+		redirect(303, '/notifications');
+	}
+};